1. Field of the Invention
The present invention relates to a magneto-optical device using the Faraday effect, and to an optical magnetic field sensor probe for detecting a magnetic field by using the same to measure the intensity of the magnetic field.
2. Related Art of the Invention
As a method of measuring the magnetic field intensity generated around an electric current by using light, an optical magnetic field sensor combining a magneto-optical device having the Faraday effect and optical fiber is known. Such an optical magnetic field sensor provides high insulation and is free from the effects of electromagnetic induction noise, and owing to such advantages, already, it is realized as a sensor for detecting accidents of high voltage distribution lines in the electric power field (Journal of the Institute of Electrical Engineers of Japan, Section B, Vol. 115, No. 12, p. 1447, 1995). Recently, moreover, there is a mounting need for higher performance for this instrument, and an optical magnetic field sensor of high precision and small size is demanded.
As the optical magnetic sensor making use of the Faraday effect, hitherto, the sensor heads as shown in FIGS. 16(a) and 16(b) have been disclosed (see Journal of Japan Society of Applied Magnetics, Vol. 19, No. 2, p. 209, 1995, and IEEE Transactions on Magnetics, Vol. 31, p. 3191, 1995). In FIGS. 16(a) and 16(b), magneto-optical devices 1 of rare earth iron garnet material are disposed in a magnetic field H. The sensor head in FIG. 16(a) constitutes a collimated optical system using collimated lenses 24a, 24b. Herein, the rare earth iron garnet material used as the magneto-optical device 1 measures 3 mm square and 60 .mu.m in film thickness. Optical fibers 6a and 6b are multi-mode optical fibers with a core diameter of 200 .mu.m. In a polarizer 2 and an analyzer 3a, polarizing beam splitters of a 5 mm cube are used, and the polarizer 2 and analyzer 3a are disposed so that the direction of polarization may be mutually different by 45 degrees. The light entering from an input optical fiber 6a is transformed into a parallel light beam by the collimated lens 24a. It is further transformed into a straight polarized light beam by the polarizer 2, and passes through the magneto-optical device 1, and the plane of polarization is rotated in proportion to the intensity of the magnetic field by the Faraday effect. The rotated straight polarized light passes through the analyzer 3a different by 45 degrees in the transmission and polarization direction with respect to the polarizer 2, and is reflected by a total reflection mirror 4, condensed by the collimated lens 24b, and is focused on the output optical fiber 6b. In such an optical system, the analyzer 3a is fixed, the light output from the polarizer 3a is utilized in one port only, and hence it is called the non-differential fixed analyzer method, in which the change in the magnetic field intensity is converted into a change in quantity of light so as to be measurable. In the optical magnetic field sensor shown in FIG. 16(a), of the light diffracted by the multiple-domain structure of rare earth iron garnet material serving as the magneto-optical device 1, only the 0th-order light is received, and therefore it is hitherto unveiled that the increases as the magnetic field becomes higher.
On the other hand, the sensor head in FIG. 16(b) constitutes a confocal optical system using spherical lenses 25a, 25b as the lenses, and forming a beam waist at the position of the magneto-optical device 1. Thus, the light diffracted by the rare earth iron garnet material can be received up to a high order, so that the linearity is improved. In FIG. 16(b), in order to shorten the optical path length so as to form a beam waist at the position of the magneto-optical device 1, a 3 mm square glass polarizing plate is used in the analyzer 3b. The spherical lenses 25a, 25b, are 3 mm in diameter, being made of material BK-7 with a refractive index of 1.517, and the sensor head measures 12 mm in width and 20 mm in length. These optical magnetic field sensors are installed in the gap of an iron core 16 as shown in FIG. 9 (a block diagram of an optical transformer using the optical magnetic field sensor probe of the invention), and used as optical transformers. Therefore, the smaller the width of the sensor head, the narrower the gap that may be formed, so that an optical transformer of high sensitivity may be realized.
As the magneto-optical device 1 used in such a sensor, the rare earth iron garnet material as shown in formula 2 is disclosed (see Technical Research Report of Electronics, Information and Communication Society of Japan, OQE92-105, 1992). In this prior art, by replacing Y with Bi or Gd, a magneto-optical device of excellent temperature characteristic is realized. The chemical formula of the crystal used in this prior art is shown in formula 2. EQU Bi.sub.1.3 Gd.sub.0.1 La.sub.0.1 Y.sub.1.5 Fe.sub.4.4 Ga.sub.0.6 O.sub.12(Formula 2)
The linearity and temperature characteristic of the optical magnetic field sensor shown in FIG. 16(b) fabricated by using this magneto-optical device are shown in FIG. 17 and FIG. 18. As shown in FIG. 17, a favorable linearity of 1.0% or less is realized in a magnetic field range of about 25 Oe to 300 Oe. However, to measure a weak magnetic field of less than 25 Oe, the linearity error is large, and a practical problem is noted. The measuring range is narrow, only up to 300 Oe, and an optical magnetic field sensor having a wider measuring range is desired. FIG. 18 shows the result of measuring changes of sensitivity depending on temperature by using two kinds of sensor optical systems, that is, the collimated optical system shown in FIG. 16(a) and the confocal optical system shown in FIG. 16(b), by using the magneto-optical device shown in formula 2. The change rate of sensitivity is normalized by room temperature, and the applied magnetic field is an alternating-current magnetic field of 50 Oe and 60 Hz. In the optical magnetic field sensor shown in FIG. 16(a) composed of the collimated optical system for receiving 0th-order diffracted light only as indicated by bullet marks in FIG. 18, the temperature dependent change of sensitivity of 1.0% or less is obtained. However, in the case of using the magneto-optical device shown in formula 2 in the optical magnetic field sensor shown in FIG. 16(b), a positive characteristic of about 10% of temperature dependent sensitivity change rate is shown as indicated by blank circle marks in FIG. 18. That is, the optical magnetic field sensor in FIG. 16(b) is excellent in linearity, but has a serious problem in the temperature characteristic of the sensitivity.
Therefore, in the prior art, an optical magnetic field sensor satisfying the contradictory problems of favorable linearity and favorable temperature characteristic cannot be realized. Accordingly, an optical magnetic field sensor of smaller size and higher precision is demanded.